Stopcock is a plug with a through hole as the opening and closing pieces, through the plug body and stem rotation to achieve the valve opening and closing action. Plug valve structure is simple, quick opening and closing, fluid resistance. This article made some improvements and calculations of the strength of plug valves used in drilling. Plug the opening and closing member is a perforated cylinder, around the axis of rotation perpendicular to the channel, so as to achieve the purpose of opening and closing channels. Plug valves are mainly used to open and close the pipeline and equipment media purposes. Stopcock valve main advantages are: For regular operation, opening and closing fast, lightweight. Small fluid resistance. Simple structure, relative size, light weight, easy maintenance. Sealing performance is not limited by the installation direction, the flow of media can be arbitrary. No vibration, low noise. However, the plug valves used in drilling often appear after closing the problem is not easy to open, this paper conducted a series of discussions and improvements. 1 under the action of three kinds of given load, the tensile stress under the highest stress, the least safety factor. Therefore, the tensile strength under stress conditions is the main contradiction. 2 Stretch to the common plug valve material 40CrMnMo calculation, the safety factor of 1.71> 1.5, to meet the strength requirements. 3 Tensile load knob hole at the mouth of the stress concentration, stress concentration point have reached the yield limit of the material, according to the new strength criteria, the maximum stress concentration point 935.75MPa 4 in the groove when the inner groove The benchmark stress and stress are also large, but can meet the strength requirements. In the design should pay attention to reduce the stress concentration here. 5 Given the torsional and internal pressure loads, the safety factor for each of the critical sections is large, regardless of their strength.